Regulation of the catalytic selectivity of rhodium for the industrially important hydrogenation of 1,3-butadiene to n-butenes has been achieved by controlling the structure of essentially molecular rhodium species bound to supports. The selectivity for n-butene formation increases as the nuclearity of the metal species decreases from several Rh atoms to one, but these catalysts form the undesired product n-butane, even at low diene conversions. The n-butene selectivity increases when the rhodium is selectively poisoned with CO ligands, and it is highest when the support is the electron-donor MgO and the rhodium is in the form of clusters that are well approximated as dimers. The electron-donor support is crucial for stabilization of the rhodium carbonyl dimer sites, as it limits the oxidative fragmentation of the clusterswhich is facilitated when the support is HY zeolite (a poor electron donor)that leads to decreased catalytic activity and selectivity. The selective MgO-supported rhodium carbonyl dimers suppress the catalytic routes that yield butane, limiting the activity for H2 dissociation to avoid butane formation via primary reactions and also favoring the bonding of 1,3-butadiene over butenes to limit secondary reactions giving butane. With this catalyst, selectivities to n-butene of >99% were achieved at 1,3-butadiene conversions as high as 97%. This selectivity matches that of any reported for this reaction, and the catalyst works under milder conditions (313 K and 1 bar) than others that are selective for this reaction.

The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface-mediated reaction involving [Rh(C(2)H(4))(2)] species and H(2). X-ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal-support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58-fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species.

A gateway to partial hydrogenation: Supported rhodium species with an essentially molecular character were precisely designed for the selective partial hydrogenation of 1,3‐butadiene to give n‐butenes, transforming a metal that is traditionally considered unselective into one that is highly selective.

Extremely small supported rhodium clusters were prepared from rhodium complexes on the surfaces of solids with contrasting electron-donor properties. The samples were characterized by infrared and extended X-ray absorption fine structure spectroscopies to determine the changes occurring in the rhodium species resulting from treatments in hydrogen. Rhodium cluster formation occurred in the presence of H2, and the first steps are controlled by the electron-donor properties of the support--which acts as a ligand--and the other ligands bonded to the rhodium. The cluster formation begins at a lower temperature when the support is zeolite HY than when it is the better electron-donor MgO, provided that the other ligands on rhodium are ethene. In contrast, when these other ligands are CO, the pattern is reversed. The choice of ligands including the support also allows regulation of the stoichiometry of the surface transformations in H2 and the stability of the structures formed in the presence of other reactants. The combination of MgO as the support and ethene as a ligand allows restriction of the rhodium cluster size to the smallest possible-and these were formed in high yields. The data presented here are among the first characterizing the first steps of metal cluster formation.